The preparation of metal complexes from azobarbituric acid with nickel salts and the use thereof as yellow pigments has long been known and has been described many times in the literature (cf., for example, W. Herbst, K. Hunger: Industrial Organic Pigments, 3rd edition 2004, p. 390/397). It is additionally known that these products can be reacted further, for example with melamine or melamine derivatives, in order to improve the performance properties of the pigments, for example in the colouring of plastics, lacquers and colour filters for LCDs.
In addition, literature states that colouristic properties can also be adjusted using, apart from nickel salts, one or more salts of different metals as well. The application EP-A 1 591 489 describes metal complexes of azo compounds containing, as metals, those from the group of the alkali metals, alkaline earth metals, the lanthanoids, and aluminium, scandium, titanium, vanadium, chromium, manganese, cobalt, copper, nickel and zinc, and optionally iron. The pigments obtained have a different colour locus compared to the pure nickel-azobarbituric acid complexes.
Controlled surface coverage of the metal azo pigment can likewise achieve an improvement in performance-based properties, specifically the lowering of the dispersion hardness as a measure of the dispersion properties of the pigment. However, this method of improving dispersibility is associated with a reduction in the colour intensity of the pigment, which is directly dependent on the concentration of covering agent.
A further means of adjusting performance-based properties is to subject the pigments produced from nickel-azobarbituric acid complexes with melamine, for example, to a heat treatment. This process step is associated with a controlled alteration in the particle size of the pigments and the specific surface area thereof. This process is described, for example, in EP-A 0 994 162.
However, the metal azo pigments known from the prior art are still in need of improvement with regard to the performance properties thereof.
Furthermore, for environmental reasons, there is the pressing desire to provide metal azo pigments having a minimum content of nickel ions or even entirely nickel-free products for these applications.